root password and security

https://help.ubuntu.com/community/RootSudo
Hello! I've read the RootSudo article, but I'm not quite understanding it well enough. Is it secure that root user has no password by default? Should I activate root account and give it a strong password for more security?

But when I type "su" command, it asks for the root password which is different from my account's password. What is root password when root is disabled? A hash or something?
When the root account is disabled, does it have empty password?

There is no need for 'su'. To get a root terminal with 'sudo':$ sudo -i
To keep most of the current environment (e.g., keep the current value of $HOME, do not 'source'/root/.profile, etc.), you may prefer:$ sudo -s